Thread breakage detector

ABSTRACT

A magnet is mounted in a housing of a thread breakage detector and has an edge bounding a surface. An arm composed at least in part of magnetizable material is fulcrumed on this edge for pivotal movement and has a portion which is displaceable by magnetic attraction from a position remote to a position proximal to the surface of the magnet. An electrical circuit is provided, having at least two contacts at least one of which is carried by the portion of the arm so as to engage the other contact when the portion is proximal to the surface of the magnet, and tensiondetecting means in form of a separate rockable lever, or a portion of the arm itself, is provided which cooperates with a tensioned thread and maintains the portion of the arm in its position remote from the surface of the magnet while the thread exerts a predetermined tension.

[ THREAD BREAKAGE DETECTOR Filed:

[75] Inventors: Karel Mikulecky, Chocen; Jiri Elias, Brandys Nad Orlici;Frantisek Filip, Usti Nad Orlici, all of Czechoslovakia Assignee:Vyzkumny Ustav Bavlnarsky, Usti Nad Orlici, Czechoslovakia Oct. 26, 1971Foreign Application Priority Data Nov. 6, 1970 Czechoslovakia ..7459/70US. Cl. ..57/8l, 57/58.89, ZOO/61.18 Int. Cl ..D0lh 13/16, B65h 63/02Field of Search ..57/80, 81, 58.89-58.95;

References Cited UNITED STATES PATENTS 2/1964 Rudd ..200/6l.18 4/1967Pesek et al. ....200/61.18

. 7/1972 Santerre et a1. ..57/81 [451 Apr. 17, 1973 FOREIGN PATENTS ORAPPLICATIONS 373,766 4/1923 Germany ..200/6l.l8

Primary Examiner-John Petrakes AttorneyMichael S. Striker [57] ABSTRACTA magnet is mounted in a housing of a thread breakage detector and hasan edge bounding a surface. An arm composed at least in part ofmagnetizable material is fulcrumed on this edge for pivotal movement andhas a portion which is displaceable by magnetic attraction from aposition remote to a position proximal to the surface of the magnet. Anelectrical circuit is provided, having at least two contacts at leastone of which is carried by the portion of the arm so as to engage theother contact when the portion is proximal to the surface of the magnet,and tension-detecting means in form of a separate rockable lever, or aportion of the arm itself, is provided which cooperates with a tensionedthread and maintains the portion of the arm in its position remote fromthe surface of the magnet while the thread exerts a predeterminedtension.

14 Claims, 6 Drawing Figures PATENTED APR 1 H973 sum 1 or 2 PATENTED APRI 7 I973 SHEET 2 OF 2 THREAD BREAKAGE DETECTOR BACKGROUND OF THEINVENTION The present invention relates generally to a thread breakagedetector, and more particularly to a thread breakage detector which isusable with particular advantage, although not exclusively, in anopen-end spinning machine.

It is already known that open-end spinning machines chamber. Suchmachines are conventionally provided I with devices which automaticallyrepair thread or yarn breakage, in order to eliminate the need for anoperator to carry out such a repair manually. In order to be able toeffect proper automatic repair of such a breakage, however, it isessential that the breakage be reliably and rapidly detected when itoccurs because otherwise the thread repair device cannot respond rapidlyenough. Such repair devices require that the direction of the piece ofthread which is downstream of the breakage be reversed, so that thebroken end be returned back into the cavity of the rotary spinningchamber for link-up with the thread still located therein. This,however, is possible only if the broken end of the yarn is made toreverse its direction of movement before it has left the withdrawingchannel through which it is withdrawn out of the rotary spinningchamber.

The problem in question has been vexing to the industry for some timeand various remedies have been proposed.'Thus, there exists a proposalto provide a channel through which air is directed in counterflow to themovement of the thread out of the spinning chamber, in order to carrythe broken thread-end back into the spinning chamber and deposit it onthe fibercollecting surface thereof for link-up with the thread endstill located in the spinning chamber. Other solutions have also beenproposed. However, none of them will provide the desired link-up if thedevices for this purpose are not actuated rapidly enough upon theoccurrence of a thread breakage. Unless such rapid actuation occurs, thebroken yarn end will escape from the yarn or thread-withdrawing channeland it is then necessary to effect the link-up manually.

The automatic thread breakage repairing devices are usually constitutedof a mechanism utilizing gear means,'brake means and clutch means and ifthread breakage occurs, the brake must first stop the withdrawal motionof the yam-withdrawing and the yarn wind-up elements of the machine,whereupon the I clutch means reverses the direction of rotation of thesedevices for a predetermined time period until the trailing broken threadend has been returned back into the rotary spinning chamber. Suchdevices must, of course,

be triggered in the event of thread breakage, and for this purposethread breakage detectors areprovided which constantly supervise thethread and initiate a signal when they detect a breakage of the thread.Thread breakage can, as is well known in the art, be the result of manyfactors, for instance the existence of burrs or other impurities in thefibers which are being spun in the rotary spinning chamber into a yarnor thread.

A plurality of different types of thread breakage detectors are alreadyknown from the prior art. They operate basically on one of twoprinciples, namely to detect either a reduction in the tension of thethread being withdrawn, or a loss of tension. The latter is determinedby the action of centrifugal forces exerted upon the yarn and in thespinning chamber, as well as upon fibers deposited on thefiber-collecting surface of the spinning chamber in preparation fortwisting into a yarn or thread. A common problem of the prior-art threadbreakage detectors, aside from their complexity and thus theirexpensiveness, is the fact that they have moving parts in whichrelatively large masses must be accelerated when the detector detectsthe breakage of a thread. This means that these moving parts haverelatively high inertia and consequently the thread breakage detectorsare characterized by having a relatively long response time, that is thetime between detecting the existenceof a break in the thread and theresponse of the detector to such a discovery. Generally speaking, theprior-art devices for detecting thread breakages have been foundsuitable only for low yarn or thread withdrawal speeds, for instancespeeds of approximately 30 meters per minute. One device which has beenfound effective in applications where the thread withdrawal speed isrelatively slow, and which requires the acceleration of relatively smallmasses and is simple in its construction, is for instance disclosed inCzechoslovak Pat. No. 130,260. It utilizes a rocking lever a baseportion of which is received in a notch of a stationary support so thatit can pivot about its own edge, and the base portion or the support isa magnet, with the thread acting upon another portion of the rockinglever. The problem even with this otherwise effective thread breakagedetector is the fact that still a considerable inertia to movement ofthe base portion must be overcome. Consequently, although this type ofdetector is effective for thread withdrawal speeds which are relativelylow, experiments have shown that it cannot be effectively used if thethread withdrawal speeds are relatively high, such as for examplebetween approximately 60 and 120 meters per minute.

In this connection it is emphasized that the desired very brief responsetime, that is the time required between detection of thread breakage andclosure of the contacts of an electrical circuit which will triggeroperation of the thread repair device, is dependent upon the threadwithdrawal speed, the length of the withdrawal channel and the innerdiameter of the spinning chamber. The length of the withdrawal channelis usually about mm; if the yarn withdrawal speed is increased, then thelength of the withdrawal channel must of course similarly be increasedif the response time of the thread-breakage detector cannot be improved.On the other hand, it is well known in the industry that it is desiredto make the thread withdrawal channel as short as possible because itslength influences the overall height of the spinning unit.

Also, considering the relationship between force and mass, that is thatforce equals mass multipled by the acceleration (F m'a), it will beappreciated that in order to obtain the acceleration required for agiven purpose the weight of the moving mass must be reduced as muchaspossible. The force is determined by the yarn tension whereas thecontact closing time of the thread breakage detector is determined bythe length of the withdrawal channel and by the yarn withdrawal speed.

Assuming, for the sake of example, a withdrawal speed of 120 meters perminute, it is necessary that the contacts of the electric controlcircuit which activates the thread breakagerepair device, be closedimmediately after the thread breakage has occurred, that is after thetrailing yarn end of the tom-off portion of the yarn or thread hastraversed a path of approximately 20-30 mm from the time the breakagehas occurred. This corresponds to a response time interval of betweenand milliseconds. After the contacts have been closed, the brakes engagethe yarn take-off and yarn wind-up devices, braking them, and thereuponthe clutches will reverse the operation of the yarn take-off and yarnwind-up devices for a certain period of time until the trailing end ofthe broken-off yarn or thread portion has been returned into thespinning chamber.

Because of the considerable inertia of the masses which must be brakedin the yarn take-off and yarn wind-up devices, and which must then begiven reverse motion, thisoperation will consume additional time and thetrailing yarn end will move in the withdrawal channel by a distance ofanother 40-60 mm for a period of between and 30 milliseconds. Only thenwill the trailing yarn. end be stopped and begin its 5 jects which willbecome apparent here after, one feature of the invention resides in athread breakage detector, particularly for use in open-end spinningmachines, which comprises a housing and magnet means mounted in thehousing and having an exposed surface bounded by an edge. An arm iscomposed at least in part of magnetizable material and is fulcrumed onthe edge for pivotal movement there about, having a return movement tothefiber-collecting surface of the rotary spinning'chamber. Evidently,the time periods here involved are extremely short, and it has beenfound that the known thread breakage detectors are notv capable torespond to the detection of thread breakage with a speed sufiicient toassure stoppage and reversal of motion of the trailing yarn end whilethe latter is still located in the yarn withdrawal channel.

The known thread breakage detectors have an additional disadvantage inthat they cannot be adjusted to various yarn tension values independence on the yarn strength, the types of fibers being spun, and therevolutions of the rotary spinning chamber.

1 ,SUMMARY OF THE INVENTION It is, accordingly, an object of the presentinvention to overcome the disadvantages outlined above with respect to,the prior art.

More particularly it is an object of the present invention to provide animproved thread breakage detector thread breakage detector which issimple in its construction and reliable in its operation.

It is another object of the invention to provide such a detector whichis not subject to a deterioration of its efficiency by exposure toexternal influences, such as dust deposits or the like.

portion displaceable by magnetic attraction from a first to a secondposition in which it is respectively farther from and closer to thesurface of the magnet means.

Electric circuit means is provided and includes at least" two contactsone of which is carried by the arm and which are adapted for engagementin response to displacement of the portion of the arm from first tosecond position. Finally, tension-detecting means is provided, beingadapted for cooperation with a tensioned thread and operative formaintaining the portion of the arm in the first position while thedetecting means is subjected by the thread to a predetermined tension.

The tension-detecting means may be, according to one embodiment of theinvention, a rocking arm a free end of which is engaged by the thread,whereas another end contacts a displaceable pinwhich bears upon theportion of the fulcrumed arm and normally maintains it in the firstposition counter to themagnetic attraction while tension is exerted bythe thread. Advantageously, the rocking arm or lever is arranged tofreely rock in a cavity provided in the withdrawal body of an open-endspinning machine, having at one end a sharp edge received in a matingrecess in the cavity and pivotable about or rockable about this sharpedge. The cavity in turn can communicate, via a duct provided adjacentthe aforementioned recess, with the yarn withdrawal channel, or with theambient atmosphere, so that any dust which might tend to settle into therecess and could disadvantageously influence the freedom of rockingmovement and therefore the quickness of response of the detector, can bewithdrawn through this duct. I 7

It is also possible, according to another embodiment of the invention,for the tension-detecting'means to be constituted by a terminal sectionof the arm itself. In this case the opposite terminal section maybeprovided with a depression or groove in which the edge bounding theexposed surface of the magnet means is received. The magnet means itselfis according to one embodiment a single unitary and-stationary permanentmag- .permanent magnets one of' which is stationarily mounted whereasthe other is movable towards and away from 'it in longitudinal directionof the arm. Finally, the position of at least one of the contacts withreference to the other can also be made adjustable, for which purposeone embodiment of the invention discloses appropriate means.

The novel features which are considered as characteristic for the.invention are set forth in particular in the appended claims. Theinvention itself, however,

- both as to its construction and its method of operation,

together with additional objects and advantages thereof, will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a somewhat diagrammaticsectional illustration showing one embodiment of the invention innonoperated condition;

FIG. 2 is a view similar to FIG. 1 showing another embodiment of theinvention in operated condition;

FIG. 3 is a view similar to FIG. 1 illustrating a further embodiment ofthe invention;

FIG. 4 is a view similar to FIG. 3 showing an additional embodiment ofthe invention;

FIG. 5 is a view similar to FIG. 1 illustrating still a furtherembodiment of the invention; and

FIG. 6 is an elevational view of FIG. 5 looking towards the right-handside and with portions omitted for the sake of clarity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Discussing the drawing now indetail, and firstly the embodiment illustrated in FIG. 1, it will beseen that the novel thread breakage detector is located at or on a bodyor housing 1 of a spinning machine which is otherwise not illustrated indetail because it does not form a part of the invention. The body 1 hasa yarn or thread withdrawal channel 2 which communicates with thenon-illustrated spinning chamber and through which the thread or yarn 3is withdrawn, in the illustrated embodiment in upward direction.

In FIG. 1, however, the channel or duct is omitted and it will of coursebe understood that in such a case appropriate steps should be taken toprevent the intrusion of dust or similar contaminants into the cavity 6,for instance by appropriately sealing it or shielding it. In any case,the lever 5 alternately closes and opens the contacts of an electriccircuit, depending upon its posi- FIG. 1 shows the novel thread breakagedetector 4 which is provided for detecting breakage of the thread 3 andwhich has a housing 12 located adjacent to a recess or cavity 6 providedin the body 1. A rocking lever 5 is also arranged in the recess 6 andhas a lower end portion 7 provided with a sharp edge which is receivedin a corresponding recess 8 provided in the bottom wall bounding thecavity 6 so that the lever 5 can rock about this sharp edge in thedirection of the double-headed arrow. The upper free end portion 9 ofthe lever 5 is engaged by the thread 3 which when tensioned tends todisplace and urge the lever 5 in clockwise direction.

It is appropriate to discuss at this point the embodiment of FIG. 2,because it differs from that of FIG. 1 only in the provision of achannel or duct 10 which communicates with the ambient atmosphere at oneend, in FIG. 2 by means of the channel 2 which is in turn incommunication with the ambient atmosphere, whereas the other end of theduct or channel 10 communicates with the cavity 6 in the region of therecess 8. The purpose of this arrangement is to prevent the depositionof dust or similar contaminantswhich enter the cavity 6 from above-inthe recess 8. The communication of the channel 10 with the channel 2,rather than directly with the ambient atmosphere, is currently preferredbecause in this manner a continuous air flow prevailing in the channel 2and directed inwardly. towards the non-illustrated rotary spinningchamber (that is downwardly in FIG. 2) effects permanent withdrawal ofdust from the cavity 6.

tion, that is depending upon whether it is urged towards the right byengagement with the tensioned thread 3 as shown in FIG. 1, or whether inthe absence of tension by the thread 3 the contacts can close and thelever 5 is displaced towards left as shown in FIG. 2. In FIGS. 1 and 2the remainder of the embodiments is identical. The housing 12 may beformed as an integral part of the body 1 or it may be a separate unitwhich is appropriately securedfor instance removablyto the body 1.Located in the housing 12 is a magnet 20 which is fixedly mounted and asurface of which is designated with reference numeral 25 and bounded byan edge 19. An arm 18 is fulcrumed on the edge 19 in the illustratedembodiment by the portion 21 of the arm 18 being bent at an angle to theremainder of the arm so as to provide a recess 22 in which the edge 19is located. The arm 18 is attracted by the magnetic force of the magnetto the surface 25; it carries one contact 17 of an electrical circuit 14the other contact 13 of which is fixedly mounted in the housing 12. Itis important that the weight of the arm 18 be as low as possible becausethe response time, that is the period required for closing or openingthe circuit 14, depends to a large extent on this weight. The width ofthe arm 18 in direction normal to the plane of FIG. 1 correspondspreferably to at least a substantial extent to that of the chamber 1 1in the housing 12, seen in the same direction. This prevents the arm 18from moving to a position in which it could become wedged in the chamber11 and blocked from performing the necessary movement.

The housing 12 is formed with an aperture communicating with the cavity6 and in which there is located a pin 24 which can slide longitudinallyof the aperture and bears with its opposite ends against the lever 5 andagainst an end portion 23 of the arm 18, respectively. When the portion9 of the lever 5 is engaged by thread 3 under tension, the lever ispivoted towards the right in clockwise direction by this tension exertedby the thread 3, pushing the pin 24 also towards the right anddisplacing the arm 18 via the pin 24 towards the right against themagnetic force exerted upon the arm 18 by the magnet 20. In thiscondition the detector does not operate the non-illustrated signal orthread breakage repair devices.

When, however, the thread 3 breaks, then the pore 17 engage and completethecircuit 14.

As shown in FIG. 3, the lever 5 can be omitted and a terminal section ofthe arm 18 itself can constitute the equivalent of the lever 5, beingengaged by the thread which then moves over the terminal section of thearm 18 in the same way in which it moves over the terminal section 9 ofthe lever 5 in FIGS. 1 and 2. In this embodi ment the cavity 6 and thepin 24 can of course be omitted, "together with the lever 5.

Anembodiment similar to that of FIG. 3 is illustrated in FIG. 4 whereagain a terminal section of the arm 18 is engaged by the thread. In FIG.4, however, it is illustrated that the arm 18 can also carry twocontacts, and that there may beprovided two stationary contacts each ofwhich cooperates with one of the contacts carried bythe arm 18.Moreover, in this embodiment the edge constituting a boundary of thesurface 25 where the same adjoins the upper surface 26 of the magnet 20an arrangement which is the simplest in terms of the manufacture andwhich is fully satisfactory-can be replaced with an edge 19 provided ona projecting ridge extending beyond the surface 25; in this case theportion '21 of the arm 18 is configurated asshown in FIG. 4 in order toprovide the recess 22 in which the edge 19 of the projection is againlodged.

The magnet 20 need not of course be a permanent magnet, but couldinstead be an electromagnet. Additionally,however, it need not be of onepiece, such as the permanent magnets shown in FIGS.'14. Instead, FIGS. 5and. 6 illustrate an embodiment in which the magnet consists of twopieces in order to afford suitable adjustment of the magnetic attractionacting on the arm 18. In FIG. 5 like reference numerals identify thesame elements as in the preceding embodiments. Here, however, thestationary magnet has a companion piece, namely a magnet 27 which can bemade to move towards and away from the magnet 20 longitudinally of thearm 18, as indicated by the double-headed arrow associated with themagnet 27. How this is accomplishedisobvious to those skilled in the artand various possibilities will offer themselves. We have chosen toillustrate one particular approach, namely to provide the outside of onewall-of the housing 12 with a plurality of teeth or serrations 12b, andto provide a slide member 28 a pointer 28a of which is engaged in theserrations 12b. In suitable manner, as by a spring-loaded rod or thelike, the pointer 28a is in connection with the magwith a stationarycontact 13, so that both extreme'positions' of the arm 18 can beutilized for controlling a thread breakage repair device or the like.

n In operation of the novel thread breakage detector the normal tensionto which the yarn 3 is'subject during withdrawal, as a result ofcentrifugal force prevailing in the non-illustrated rotary spinningchamber, will cause I the lever 5 or the arm 18 to be displaced to aposition in which the contacts 13 and 17 are out ofengagement. However,as soon as the thread 3 breaks, the tension of the thread will dropimmediately to zero and the arm l8--which has low weight and mass-willbe immediately attracted towards the surface 25 of the magnets 20 or 20,27. In the embodiment of FIGS. 1, 2 and 5 it will at the same time shiftthe pin 24 towards the left and rock the lever 5 in counterclockwisedirection. In any case, however, the contacts 13 and 17 will engage andcomplete the electric circuit l4'which can thus operate thenon-illustrated thread breakage repair device before the trailing end'ofthe yarn 3 has an opportunity to escape from the passage 2.

It is an essential characteristic and advantage of the thread breakagedetector according to" the present invention that the mass to be moved,that is the lever 5 and the arm 18, is reduced to a considerable extentso that the detecting operation and the closure of the circuit 14issignificantly accelerated because of the lower I the prior art, andfor instance with respect to the apnet 27 via the slot 28b which isshown in phantom lines,

i with the rod being illustrated also in phantom lines anddesignated'with reference numeral 28c (see FIG. 5). Because of thespring-loading of this arrangement the portion 28 canbe drawn outwardlyto disengage the pointer 28a from the teeth or notches 12b, whereuponthe magnet 27 can be moved upwardly or downwardly along the slot 28b bymoving the pointer or member 28. With this arrangement the magnet 27 issecured against undesired displacement but, on the other hand, theembodiment provides for lengthening or shortening the magnetic forcefield R which acts relative to the edge 19 upon the arm I8, so as toadjust the force with which the arm 18 is attracted to the magnets 20,27.

The wall 12a of the housing 12 is provided with an additional bore, thatis a bore in addition to that in which the slidable pin 24 isaccommodated, and the contact 13 is mounted in this bore 15-which may beinternally threaded-and can be locked in place by a nut or similar means16. Thus, the contact 13 can be moved inwardly or'outwardly with respectto the housing 12, and therefore with reference to the contact, 17.

However, the feature of mounting the contact 13 displaceably canbeomitted in the embodiment of FIG. 5, and in any case the adjustment ofthe magnetic force paratus disclosed in the earlier-mentionedCzechoslovakian Pat. No. 130,260 the response time of the novel threadbreakage detector is approximately three times shorter. The noveldetector can evidently respond to thread breakages with sufficientspeed, even when the thread or yarn withdrawal speedsrare extremelyhigh.

It is a further advantage of the present invention that the force actingon the arm 18 can be adjusted, so that it is possible to take intoaccount the strength of the type of yarn or thread being spun. This'isimportant because the tension on the thread, and therefore the forcerequired of the magnetic attraction upon the arm 18, is greater in thecase of stronger yarns, and vice versa. Also, the construction isparticularly simple and capable of rapid response times because boththe'arm l8 and the lever 5 are mounted not by means of pivots or othersimilar means, but exclusively pivot about sharp edges on which they aresupported, thereby reducing friction to a minimum and, of course,greatly simplifying the construction.

It will be understood that each of the elements described above, or twoor moretogether, may also find a useful application inother types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in athread breakage detector, it

is not intended to be limited to the details shown, since variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting features thatfrom the standpoint of prior art fairly constitute essential features ofthe generic or specific aspects of this invention and, therefore, suchadaptations should and are intended to be comprehended within themeaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent isset forth in the appended claims:

1. A thread breakage detector, particularly for use in open-end spinningmachines, comprising a housing; magnet means mounted in said housing andhaving an exposed surface bounded by an edge; an arm composed at last inpart of magnetizable material and being fulcrumed on said edge forpivotal movement thereabout, said arm having a portion displaceable bymagnetic attraction from a first to a second position in which it isrespectively farther from and closer to said surface; electric circuitmeans including at least two contacts one of which is carried by saidarm and which are adapted for engagement in response to displacement ofsaid portion from said first to said second position thereof; andtension-detecting means adapted for cooperation with a tensioned threadand operative for maintaining said portion in said first position whilesubjected by the thread to a predetermined tension.

2. A detector as defined in claim 1, wherein said tension-detectingmeans comprises a displaceable member 4. A detector as defined in claim3, said rocking lever having a tenninal portion bounded by a sharplydefined edge, and said cavity having a wall provided with a recess inwhich said sharply defined edge is matingly received for free rockingmovement of said rocking lever about said sharply defined edge.

5. A detector as defined in claim 4; further comprising a channel insaid wall means and communicating with said cavity in the region of saidrecess for evacuation of contaminants from said cavity.

6. A detector as defined in claim 5, said channel having an inner endcommunicating with said cavity, and an outer end communicating with theatmosphere.

7. A detector as defined in claim 5; further comprising a passage forsaid thread in communication with the atmosphere; and wherein saidchannel has an inner end.

communicating with said cavity, and an outer end communicating with saidpassage.

8. A detector as defined in claim 1, wherein said tension-detectingmeans is a free end portion of said am.

9. A detector as defined in claim 1, said arm having a terminal sectionadjacent said edge and provided with a receiving recess in which saidedge is matingly received.

10. A detector as defined in claim 1, said housing having an interiorchamber of predetermined width in which said magnet means and said armare accommoda ted; and wherein said arm has a transverse dimension indirection of said width which substantially corresponds to the latter. 1

11. A detector as defined in claim 1, wherein said magnet meanscomprises a single permanent magnet.

12. A detector as defined in claim 1, wherein said magnet meanscomprises a stationary first magnet member and a second magnet memberwhich is shiftable toward and away from said first magnet member indirection longitudinally of said arm.

13. A detector as defined in claim 1, said magnet means having anelongated ridge projecting beyond the plane of said surface and beingprovided with said edge.

14. A detector as defined in claim 1; and further comprising adjustingmeans for adjusting the position of at least one of said contacts withreference to the other contact.

1. A thread breakage detector, particularly for use in open-end spinning machines, comprising a housing; magnet means mounted in said housing and having an exposed surface bounded by an edge; an arm composed at least in part of magnetizable material and being fulcrumed on said edge for pivotal movement thereabout, said arm having a portion displaceable by magnetic attraction from a first to a second position in which it is respectively farther from and closer to said surface; electric circuit means including at least two contacts one of which is carried by said arm and which are adapted for engagement in response to displacement of said portion from said first to said second position thereof; and tension-detecting means adapted for cooperation with a tensioned thread and operative for maintaining said portion in said first position while subjected by the thread to a predetermined tension.
 2. A detector as defined in claim 1, wherein said tension-detecting means comprises a displaceable member bearing upon said portion of said arm, and a rockable lever bearing upon said displaceable member and having a free end portion adapted for engagement with the tensioned thread, said lever urging said portion to said first position thereof via said displaceable member and under the influence of tension exerted by said thread upon said free end portion.
 3. A detector as defined in claim 2; further comprising wall means defining a cavity adjacent said housing, and wherein said lever is arranged in said cavity for rocking movement therein.
 4. A detector as defined in claim 3, said rocking lever having a terminal portion bounded by a sharply defined edge, and said cavity having a wall provided with a recess in which said sharply defined edge is matingly received for free rocking movement of said rocking lever about said sharply defined edge.
 5. A detector as defined in claim 4; further comprising a channel in said wall means and communicating with said cavity in the region of said recess for evacuation of contaminants from said cavity.
 6. A detector as defined in claim 5, said channel having an inner end communicating with said cavity, and an outer end communicating with the atmosphere.
 7. A detector as defined in claim 5; further comprising a passage for said thread in communication with the atmosphere; and wherein said channel has an inner end communicating with said cavity, and an outer end communicating with said passage.
 8. A detector as defined in claim 1, wherein said tension-detecting means is a free end portion of said arm.
 9. A detector as defined in claim 1, said arm having a terminal section adjacent said edge and provided with a receiving recess in which said edge is matingly recEived.
 10. A detector as defined in claim 1, said housing having an interior chamber of predetermined width in which said magnet means and said arm are accommodated; and wherein said arm has a transverse dimension in direction of said width which substantially corresponds to the latter.
 11. A detector as defined in claim 1, wherein said magnet means comprises a single permanent magnet.
 12. A detector as defined in claim 1, wherein said magnet means comprises a stationary first magnet member and a second magnet member which is shiftable toward and away from said first magnet member in direction longitudinally of said arm.
 13. A detector as defined in claim 1, said magnet means having an elongated ridge projecting beyond the plane of said surface and being provided with said edge.
 14. A detector as defined in claim 1; and further comprising adjusting means for adjusting the position of at least one of said contacts with reference to the other contact. 